The present invention relates to electrical heating cables, and more especially, to a device which makes it possible to ensure a good transmission of heat to elements which the cables are meant to heat.
Electrical heating cables generally comprise a conductor core of an electrically resistant material, surrounded by an insulating layer and by a covering which may comprise a metallic braiding for mechanical and electrical protection and, possibly, an external sheath. The section of the cables is round in their most common and least expensive form.
The first purpose of heating cables was the collective heating of appartments during the years 1950's. The cables were running inside protection ducts, good heat conductors, evenly lined-up on the ground and covered with a protection metallic netting. Once in place, the whole system was buried inside a concrete slab. The slab was rather thick and the cables were relatively distant from the surface so that, at the level of said surface, the heat distribution was practically uniform. The slab having considerable inertia, could be used as heat accumulating device; the cables then being supplied current solely during the non-peak hours. The protection ducts had a section appreciably larger than that of the cable, to make it possible easily to replace defective parts causing breakdowns. However, very often those breakdowns occured at points where the thermal exchange between the cable and the duct was not very good. Indeed, as the internal section of the ducts was larger than that of the cable, intimate contact between the two of them was not ensured in all points and, where the cable was not touching the duct, it would heat up and break.
Later, simpler techniques were used, which consisted in burying the cable directly inside the slabs, without any other protection than its sheath. In the case of concrete slabs, it was necessary for the concrete to be carefully vibrated so as not to let too important (air) bubbles form around the cables. Otherwise, when running through the bubbles, the cable would exagerately heat up and would in the end be ruined.
Heating cables are also used in other applications, especially to protect systems of pipes or ducts, gutters, spouts, etc., again freezing, or to maintain fluid trajectories at a desired temperature. In the case of pipes or ducts, the cable is made to run along the pipe or duct, well applied against the latter. In order to set it, rings are placed at regular intervals. The assembly may be covered with a thermally insulating coating. The round-section cable does not very well lend itself to that application, because the contact and heat-exchange zone between the cable and the pipe or duct is limited to one generatrix. In addition, in practice, there always are points on the pipe or duct where the cable becomes unglued from the wall of said pipe or duct. Those parts of the cable which are not in contact with the pipe or duct keep a large part of their heat, they become exagerately over-heated and in the end they are wrecked. Various solutions have been advanced to solve that problem. There have been conceived cables of special shape, with a part of their circumference being flat. Another cable structure consists of two non-resistant conductors placed close to each side in a tape the median part of which is resistant. Those forms of execution, although they are satisfactory, prove costly from the standpoint of fabrication Another solution consists in burying the cable inside a heat-conducting concrete (cement) cast on the surface of the pipe or duct. The execution of that process is relatively long and delicate.
In the field of apartment heating, for reasons of energy saving, the present tendency goes toward low inertia heating systems which, in addition, assure improved comfort and allow for instantaneous regulation. Heating cables are a good way of achieving them, they are unobtrusive and offer multiple laying-down solutions, especially during rehabbing: they may be placed inside mural panels, in the floors or in the ceilings, very close to the surface. There arise, however, problems of heat distribution, some hotter points developing in the areas closest to the cable, and colder points (developing) in the remote areas. That results in the production of uncomfortable zones and, possibly in damaging, as a result of the temperature differences created, the material inside which the cables are located.
In order for the cables better to diffuse their heat, it has been thought of associating with them, for specific applications, a tape element of flexible material which is good conductor of heat.
According to document U.S. Pat. No. 3 453 417, the heating cable is inserted into a metallic sleeve comprising, coming as a single part, a flat sole and an upper wall folded over said sole with median groove for reception of the cable. On one side, the sole is folded over the upper wall, so that said side presents three superposed thicknesses. The cable according to that invention is more especially planned to heat liquids, and that arrangement ensures the tightness of the sleeve which, in addition to its diffusing function, must insulate the cable from the liquid inside which it is immersed. But that is at the cost of its flexibility and no changes in direction may be imparted to it. In addition, its execution proves costly.
Document BE-A-902 275 describes a cable which also is located between a ribbon-type sole and an upper wall with a median groove for the cable's lodging. The upper wall is insulating or, as a variation, an insulating material may be provided for between said wall and the cable. In that case again, the cable is meant for special applications in which it must transfer its heat in a unidirectional manner. For a polyvalent use, the described cable presents the same drawbacks as the preceding one.